A rotor of a permanent magnetic motor of embedded magnet type (IPM motor: Interior Permanent Magnet motor) is configured in that permanent magnets are embedded into a rotor core.
In such an interior permanent magnet motor, both of magnet torque generated by magnetic fluxes of the permanent magnets and reluctance torque generated through changes in magnetic resistances (reluctances) of the rotor core can be utilized as effective torque that contributes to rotating force. An interior permanent magnet motor is accordingly a motor of energy-saving type, of high efficiency and of high torque, and is used in various industrial fields.
Cogging torque or torque ripples including such cogging torque are generated in an interior permanent magnet motor. For reducing such cogging torque and torque ripples, it is suggested to form magnetic shielding portions (details will be described later) at a rotor of a permanent magnet motor (see, for instance, Patent Literature 1).
A prior art case in which cogging torque and torque ripples are reduced by forming magnetic shielding portions at a rotor of a permanent magnet motor will now be explained with reference to FIG. 8.
In this respect, only a portion of one main magnetic pole is shown in FIG. 8, and since the other magnetic poles are of identical configuration as that of the main magnetic pole shown in FIG. 8, illustrations thereof are omitted.
Further, in FIG. 8, lines of magnetic forces showing magnetic flux flows are indicated by dotted lines.
FIG. 8 is a sectional view orthogonal to an axial direction in which a conventional rotor 1 used in an interior permanent magnet motor is shown upon extracting only a portion of one main magnetic pole.
As shown in the drawing, a rotor core 2 of the rotor 1 is a substantially cylindrically shaped member formed by laminating silicon steel plates. A motor shaft 3 is fitted into an axial core portion of the rotor core 2, and the motor shaft 3 is supported by a bearing (illustration omitted) in a freely rotatable manner.
A permanent magnet inserting hole 4 is a hole which penetrates in the axial direction from one end surface up to the other end surface of the rotor core 2. The permanent magnet inserting holes 4 are formed at the rotor core 2 at even intervals along a peripheral direction.
A plate-like permanent magnet 5 is inserted into the permanent magnet inserting hole 4 to form a single main magnetic pole. Magnetic polarities of the permanent magnets 5 are set for each of the main magnetic poles such that an outer peripheral side magnetic pole surface 5ou, of a permanent magnet 5 disposed to have an arbitrary main magnetic pole and an outer peripheral side magnetic pole surface 5ou, of a permanent magnet 5 disposed at a main magnetic pole adjoining the main magnetic pole have mutually different magnetic polarities. With this arrangement, magnetic polarities of adjoining main magnetic poles (S poles, N poles) will be different from each other.
In the rotor core 2 of the rotor 1, an axis connecting the axial core of the rotor 1 (motor shaft 3) and a center of an arbitrary main magnetic pole generating magnetic torque (a central position in a peripheral direction of the permanent magnet 5) will be a d axis of a d-q axial coordinate.
Further, from among the rotor core 2, a core between an arbitrary main magnetic pole and a main magnetic pole adjoining the main magnetic pole in the peripheral direction will be an auxiliary magnetic pole portion 6 generating reluctance torque. An axis connecting the axial core of the rotor 1 (motor shaft 3) and a central axis of the auxiliary magnetic pole 6, namely an axis orthogonal to the d axis at an electrical angle will be a q axis of the d-q axial coordinate.
Further, the rotor core 2 is formed with a magnetic shielding portion 7 which is a hole penetrating in the axial direction from one end surface up to the other end surface. The magnetic shielding portion 7 is located between the d axis and the q axis of the d-q axial coordinate at an end surface of the rotor core 2. In the example of FIG. 8, two magnetic shielding portions 7 are formed at an arbitrary magnetic pole.
Explaining the shape and the configuration of the magnetic shielding portion 7 with reference to FIG. 9, the magnetic shielding portion 7 is constituted of a main body portion 17 and an extending portion 27 succeeding to (communicating with) the main body portion 17.
The main body portion 17 contacts a q-axis side end surface 5q, which is an end surface of the permanent magnet 5 in the peripheral direction (succeeds to (communicates with) the permanent magnet inserting hole 4) and extends towards the outer peripheral surface of the rotor 1 from its axial core side portion 17-1 towards an outer peripheral side portion 17-2. However, the outer peripheral side portion 17-2 of the main body portion 17 does not reach the outer peripheral surface of the rotor 1 and the rotor core 2 resides between the outer peripheral side portion 17-2 and the outer peripheral surface of the rotor 1.
The extending portion 27 is arranged in that its base end portion 27-1 succeeds to (communicates with) the outer peripheral side portion 17-2 of the main body portion 17 and extends from the outer peripheral side portion 17-2 of the main body portion 17 along the circumferential direction towards the d axis. However, a tip end portion 27-2 of the extending portion 27 does not reach the d runs. A distance between the extending portions 27 and the outer peripheral surface of the rotor 1 (distance in the radial direction) is substantially constant at any position in the extending direction of the extending portion 27, and the rotor core 2 resides between the extending portion 27 and the outer peripheral surface of the rotor 1.
Since the magnetic shielding portion 7 is a hole (space), its magnetic permeability is remarkably smaller than that of the rotor core 2, and since it is extremely hard for the magnetic flux to pass therethrough, it functions as a magnetic shielding portion. In this respect, also when the interior of the hole (space) forming the magnetic shielding portion 7 is filled with a non-magnetic metal of low magnetic permeability (such as aluminum or brass), adhesive, varnish or resin, it is still a magnetic shielding portion.
Since such a magnetic shielding portion 7 is formed on end surfaces on both sides of the permanent magnet 5 in the peripheral direction, it has the following effects.
(1) Since the magnetic shielding portions 7 for magnetically shielding a space between an arbitrary main magnetic pole and a main magnetic pole adjoining the main magnetic pole in the peripheral direction, it is possible to reduce magnetic flux (short-circuit flux) which is generated from the outer peripheral side magnetic pole surface 5ou, of the permanent magnet 5 of the main magnetic pole, passes the auxiliary magnetic pole portion 6 of the rotor core 2 and reaches the outer peripheral side magnetic pole surface 5ou, of the permanent magnet 5 of the main magnetic pole adjoining in the peripheral direction. Since the short-circuit flux is a torque which does not cross a stator and does not contribute to generation of magnetic torque, the effective torque is increased by decreasing such short-circuit flux.
As for the reduction of the short-circuit flux, the shorter the distance between the extending portion 27 of the magnetic shielding portion 7 and the outer peripheral surface of the rotor 1 (distance in the radial direction) is and the longer the length of the extending portion 27 along the circumferential direction is, the more generation of short-circuit flux can be restrained.
(2) Since the magnetic flux generated from the outer peripheral side magnetic pole surface 5ou, of the permanent magnet 5 passes while bypassing towards a central side of the permanent magnet 5 so as to go round the magnetic shielding portion 7 of low magnetic permeability, changes in magnetic flux density distribution generated on the outer peripheral surface of the rotor 1 will become moderate by means of the permanent magnet 5. More specifically, changes in magnetic flux density distribution will become moderate at both end portions of the main magnetic pole in the peripheral direction.
In this manner, since especially changes at both end portions of the main magnetic pole in the peripheral direction from among the magnetic flux density distribution generated on the outer peripheral surface of the rotor 1 will become moderate, it is possible to reduce cogging torque and torque ripples.